tragedy strikes: --no-clean and no temp directory

[boneskull]

Expected Behavior

use with --no-clean flag doesn't throw if there's no temp dir (typically .nyc_output)

Observed Behavior

Error: ENOENT: no such file or directory, scandir '/Users/boneskull/Documents/projects/mocha/.nyc_output'
    at Object.fs.readdirSync (fs.js:910:18)
    at NYC.loadReports (/Users/boneskull/Documents/projects/mocha/node_modules/nyc/index.js:479:31)
    at NYC._getCoverageMapFromAllCoverageFiles (/Users/boneskull/Documents/projects/mocha/node_modules/nyc/index.js:412:8)
    at NYC.report (/Users/boneskull/Documents/projects/mocha/node_modules/nyc/index.js:421:18)
    at report (/Users/boneskull/Documents/projects/mocha/node_modules/nyc/bin/nyc.js:89:7)
    at /Users/boneskull/Documents/projects/mocha/node_modules/nyc/bin/nyc.js:76:27
    at ChildProcess.<anonymous> (/Users/boneskull/Documents/projects/mocha/node_modules/nyc/node_modules/foreground-child/index.js:52:5)
    at emitTwo (events.js:125:13)
    at ChildProcess.emit (events.js:213:7)
    at maybeClose (internal/child_process.js:927:16)
make: *** [test-bdd] Error 1                                    

Bonus Points! Code (or Repository) that Reproduces Issue

Well, createTempDirectory() isn't called if clean is falsy.

Forensic Information

macOS; https://gist.github.com/boneskull/fbb28823e66a439f4a7fc735cb883917

[bcoe]

@boneskull awesome thanks for the contribution 👍 will work on cutting a new release tonightish.